docs(recipes): add type hints to request_id and custom media handler snippets

[Aydeing]

Summary of Changes

Adds type annotations to a small, self-contained batch of recipe snippet files under examples/recipes/, as a step toward the rolling work in #1820:

• examples/recipes/request_id_context.py
• examples/recipes/request_id_log.py
• examples/recipes/request_id_middleware.py
• examples/recipes/request_id_structlog.py
• examples/recipes/plain_text_main.py
• examples/recipes/pretty_json_main.py

All changes are purely additive type annotations (plus an isort fix on two snippet imports that ruff check --fix produced). No runtime behavior changes; signatures only.

A docs/_newsfragments/1820.misc.rst towncrier entry is included.

Related Issues

Refs #1820

Pull Request Checklist

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• Applied changes to both WSGI and ASGI code paths and interfaces (where applicable). (N/A — middleware snippets are framework-agnostic; the recipes are not split by interface.)
• Added tests for changed code. (N/A — type-annotation-only changes to documentation snippets; no behavior to test.)
• Performed automated tests and code quality checks by running tox. (Ran ruff format and ruff check on the touched files locally — All checks passed!. AST parse + runtime import smoke-checked on Python 3.13.)
• Prefixed code comments with GitHub nick and an appropriate prefix. (No new non-trivial comments added.)
• Coding style is consistent with the rest of the framework.
• Updated documentation for changed code.
  • Added docstrings for any new classes, functions, or modules. (N/A — no new classes/functions/modules.)
  • Updated docstrings for any modifications to existing code. (N/A — signatures only.)
  • Updated both WSGI and ASGI docs (where applicable). (N/A.)
  • Added references to new classes, functions, or modules to the relevant RST file under docs/. (N/A.)
  • Updated all relevant supporting documentation files under docs/. (N/A — the recipe .rst files literalinclude these snippets unchanged.)
  • A copyright notice is included at the top of any new modules (using your own name or the name of your organization). (N/A — no new modules.)
  • Changed/added classes/methods/functions have appropriate versionadded, versionchanged, or deprecated directives. (N/A — annotations only.)
• Changes (and possible deprecations) have towncrier news fragments under docs/_newsfragments/, with the file name format {issue_number}.{fragment_type}.rst. (Added docs/_newsfragments/1820.misc.rst.)
• LLM output, if any, has been carefully reviewed and tested by a human developer. (See also: Use of LLMs ("AI").) (Annotations were drafted with LLM assistance and then each signature was reviewed against the surrounding snippet semantics, formatted with ruff, and re-validated locally.)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (e18c27f) to head (e851179).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2665   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           64        64           
  Lines         7985      7985           
  Branches      1103      1103           
=========================================
  Hits          7985      7985           

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